A preferred prior-art laser resonator type for providing a stream of laser-radiation pulses for several laser applications is a continuously-pumped, repetitively-pulsed, Q-switched, resonator. In a particularly preferred such prior-art resonator type, continuous pumping is typically provided by a plurality of laser-diodes, to a solid-state gain-medium such as Nd:YAG, Nd:YVO.sub.4, Nd:YLF or the like. Resonators including these gain-media provide fundamental laser-radiation at infrared (IR) wavelengths which, according to a particular application, can be conveniently converted to radiation at shorter-wavelengths by one or more frequency doubling or mixing steps in optically non-linear media (crystals). Such a resonator type can be efficient and compact, for example with resonator length of about 0.5 meters (m), or less. The resonator can be folded to reduce its overall physical dimension or to facilitate pumping.
Above described general advantages in efficiency, size, and wavelength-flexibility notwithstanding, any one prior-art, repetitively-pulsed laser resonator arrangement is limited in the range of pulse-parameters or pulse repetition frequencies (pulse-frequencies) which can be efficiently delivered. These ranges are determined by characteristic properties of a particular gain-medium, in particular excited state lifetime (.tau.) and gain cross-section (.sigma.). Should a potential laser application demand pulsed laser-radiation having pulse and frequency parameters which can not be efficiently provided by a known gain-medium, then the application can at best be inefficiently satisfied.
For any particular pulse-parameter or frequency range, the range of efficient power-output available may be limited by thermal-lensing in whatever gain-medium is selected. Thermal-lensing is refractive power induced in the gain-medium due to refractive-index change with temperature increase resulting from absorbed pump-power. Thermal-lensing can vary significantly with variations of absorbed power, and also with variations in pulse-frequency. While optical components of a resonator can be selected to compensate for a particular level thermal-lensing, the configuration is typically effective for only a limited range of pump-power and pulse-frequency.
There is a need for a laser resonator arrangement which overcomes above discussed limitations on laser performance imposed by gain-medium characteristics. Preferably such an arrangement should also include means for actively compensating for variations in thermal-lensing in a gain-medium.